Winding is the process of turning a flat web into a wound roll. Wound rolls are the most efficient method to store large amounts of continuous web material in a package that is convenient for material handling and shipping. The wound roll must be wound hard enough to withstand roll handling, storage conditions, clamp truck pressures, and automated material handling systems. The wound roll becomes the delivery device as the material is unwound from the roll and further processed in a manufacturing line such as in a converting process.
Although each wound roll is its own unique entity, it is a common practice in film and newspaper industries to qualify a roll as either a “hard” roll or a “soft” roll. This is done based on the “feel” or “hardness” of the wound roll. A hard roll is also commonly called a “fully compressed roll”. Typically, wound rolls of tissue, newsprint, spunbond-meltblown-spunbond laminates (SMS) fall under the category of soft rolls. Wound rolls of polyester and film laminates fall under the category of fully compressed rolls, which are so-called “hard rolls.” Also, wound rolls of low modulus films, film laminates, vertical film/filament laminates (VFL's) and stretch bond laminates (SBL's) fall under the “hard roll” category. A “hard roll” is produced when the machine direction (MD) modulus of the material is comparable to the radial modulus (ZD Modulus) of the material (Et≅Er). A “soft roll” is produced when the MD modulus of the material is much greater than the radial modulus of the material (Et>>Er).
Winding continuous web materials into a wound roll results in stored stresses within the roll, and thus winding presents an accretive stress problem. For commodity grade spunbond there is very little concern about how tightly the material is wound around the roll. However, when elastomerics, delicate laminates, or high loft web materials are wound, the roll structure (hardness) results in a permanent change of material properties inside the wound roll. This change can occur during the winding process, immediately after the winding process or over a period time.
The tension in the outermost layer of a continuous web of material being wound onto a roll is known as the “wound on tension” or “WOT.” This WOT parameter includes the web tension and any additional tension that may be due to nip load (nip induced tension), which depends on the type of winder. Each new layer added onto the winding roll during the winding process changes the stresses inside the wound roll.
Zbigniew Hakiel's paper (“Nonlinear model for wound roll stresses”, TAPPI Journal, Vol. 70(5), pp 113-117, 1987) describes how the wound roll stresses at any diametral location within the continuous web wound into a roll can be calculated given the properties (listed under “required input values”) of the roll and the material. Hakiel's paper discusses both the computational method and the flow chart for writing a computer program in any computer language, and thus a simple program can be written to predict the wound roll stresses based on what is described in Hakiel's paper. A graph of these stresses as a function of the diameter of the roll of continuous material produces a curve that exhibits a characteristic shape for both interlayer pressure (radial stress/pressure) and stresses in the machine direction (MD). The MD stress is the stress in the direction in which the web is wound onto the roll or taken off the roll and is also known as the tangential stress or the circumferential stress.
From the wound roll structure standpoint, a “soft” roll has a plateau-type radial stress profile. Addition of more web material wound on the roll does not increase the radial stresses inside these types of rolls. The only limitation to the size of the roll comes from the limitations of the winder and from the limitations of web handling, transporting units. On the other hand, a “hard” roll has a tapered radial stress profile. Addition of web material to the roll directly impacts the radial stress profile by increasing the stress inside the roll. Hence in the case of hard rolls, issues like “roll blocking” and “core crush” need to be addressed. Concern for these issues tends to restrict the size of the wound “hard” rolls.
In the case of soft rolls, the in-roll tension (also referred to as “MD stress” or “tangential stress” or “circumferential stress”) is uniform throughout the roll except very near the core and at the outside diameter. In many cases the in-roll tension is close to zero and sometimes can even be negative. In hard rolls by contrast, the thru-roll MD stress and strain produces a curve that resembles a ‘Nike®-Swoosh®’ profile. If the wound roll were to be made of high modulus film, the swoosh profile in MD strain is not a big concern as the strains are small to begin with. As the material is being unwound, this strain, typically, is quickly recovered. Hence the winding process need not undergo any modification to accommodate this stored in-roll strain.
However this is not the case in winding low modulus films, film laminates, VFL and SBL. For example, the MD modulus of VFL material is in the range of about 5 psi to about 25 psi, which is very low. The outside diameter of a wound roll of VFL material can be in the neighborhood of 62 inches. The elastomeric filaments in the VFL material make it behave like a rubber band. As anyone who has wound a rubber band around one's finger can attest, the pressures in a wound roll of VFL material are very high, even if the material is wound onto the roll at low wound on tension (WOT).
The MD stresses in rolls of such webs of material will cause the attributes (elasticity for example) of the web material on the roll to change “thru-roll,” i.e., attributes of the material wound around the core of the roll commonly will differ from the same attributes of the material wound around the outside diameter of the roll and will vary at diameters intermediate these two extreme diameters. Since the strains are very high and many materials are highly viscoelastic, the stored strains within the roll become permanent. This results in aged material properties that vary (repeatable) as a function of the roll's radius. To cope with such properties in processing the webs drawn from such hard rolls, special modifications of the process equipment (like controlled unwind) need to be in place during converting for example. The problem of coping with such properties gets complicated if printing is done on the web during converting. As the strain recovery rates are different due to different in-roll tension that the web was subjected to, the repeat length of the printed indicia may not be the same as the web material is unwound from the roll.
As noted above, webs made of elastomeric materials that are wound into rolls will experience some permanent change in the properties of the material. The elastic properties of the material wound around the core of the roll commonly will differ by more than a twenty percent variation from the elastic properties of the material wound around the outside diameter of the roll. In other words, the elastic properties “thru-roll” commonly vary by more than twenty percent. Yet the elastic properties in the machine direction (MD) are often critical to the final converting process. A change in elastic properties as the material is unwound from the roll for use in a processing line of equipment will often cause increased waste and/or downtime of the line.
Empirical studies have been conducted to develop a winding procedure that results in uniform material properties “thru-roll,” i.e., from the outside diameter to the core of the wound roll. However, conducting such studies for each differently sized new roll of differently composed material is tedious, time-consuming, and in many cases cost prohibitive.